Wind turbine with a broad-band damping device in each blade

ABSTRACT

Wind turbine comprising one or more blades, incorporating in each blade a damping device comprising a first damping element tuned at the main resonance frequency of the blade Ω o and K additional damping elements tuned at frequencies Ω  1, Ω 2 , . . . Ω k so that in the event of blade vibrations its amplitude A is reduced in a percentage Y of the amplitude Ao at the main resonance frequency of the blade Ω o in a band extended above and below the main resonance frequency of the blade Ω o in a percentage X of Ω o, said frequencies Ω  1, Ω 2 , . . . Ω k being comprised within said band.

FIELD OF THE INVENTION

The invention relates to a wind turbine with a damping device in eachblade and, more in particular, to a wind turbine with a broad-banddamping device in each blade.

BACKGROUND

Wind turbines are devices that convert mechanical wind energy toelectrical energy. A typical wind turbine includes a nacelle mounted ona tower housing a drive train for transmitting the rotation of a rotorto an electric generator and other components such as a yaw drive whichorientates the wind turbine, several actuators and sensors and a brake.The rotor supports a number of blades extending radially there from forcapturing the kinetic energy of the wind and causing the driving trainrotational motion. The rotor blades have an aerodynamic shape such thatwhen a wind blows across the surface of the blade, a lift force isgenerated causing the rotation of a shaft which is connected—directly orthrough a gearing arrangement—to the electrical generator located insidethe nacelle. The amount of energy produced by wind turbines depends onthe rotor blade sweeping surface that receives the action from the windand consequently increasing the length of the blades leads normally toan increase of the power output of the wind turbine. The blades arecontrolled to stay in autorotation regime during normal phase, and itsattitude depends on the wind intensity.

Increasing blade size has drawbacks in terms of aeroelastic response asthe general tendency is that structural damping reduces with bladelength. Due to that lessening, the dynamic response of the blade at thenatural frequencies increases proportionally to the inverse of thedamping ratio, and also depends on the proximity with the vortexshedding frequency. A typical diagram Amplitude—Relative Frequency for astructure is shown in FIG. 2 highlighting the amplitude A₀ at thefrequency of resonance F₀ and the maximum amplitude A₁ at a frequencyF₁.

Therefore it is convenient from the point of view of structuralintegrity and/or cost optimization to include somehow additional dampingin blades so that dynamic loads could be reduced. It could beparticularly interesting when by the combination of aerodynamicconditions and structural characteristic, the blades total damping isreduced (aerodynamic plus structural) and even could possibly enter in anegative aerodynamic damping values area for the relevant eigenmodes.That could happen when relevant airfoils in the blades are in stalledcondition where the lift slope is negative. This condition may happenfor stall regulated wind turbines during normal production, but also invariable pitch wind turbines when parked or in idling situations withhigh incidence angles.

There are a number of patent documents disclosing damping devices forwind turbine blades such as WO 95/21327, WO 02/08114, WO 99/32789 andU.S. Pat. No. 6,626,642.

A general drawback of those damping devices is that they are notdesigned taking into account that, for a given wind turbine model, thereis a significant variability of the structural or aerodynamic propertiesof the blades due to tolerances which involves a significant variabilityof its efficiency.

The present invention focuses on finding a solution for these drawbacks.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide wind turbines bladeswith an effective damper device in spite of the variability of theblades structural or aerodynamic properties due to tolerances.

It is another object of the present invention to provide wind turbinesblades with a damper device that allow loads reduction and/or to extendthe wind conditions that blades can stand without higher risks regardingstructural integrity.

It is another object of the present invention to provide wind turbinesblades with a damper device that allows lessening the blade weight.

These and other objects are met by a wind turbine comprising one or moreblades, each blade incorporates a damping device comprising a firstdamping element tuned at the main resonance frequency of the blade Ω_(o)and K additional damping elements tuned at frequencies Ω₁, Ω₂, . . . Qso that in the event of blade vibrations its amplitude A is reduced in apercentage Y of the amplitude A_(o) at the main resonance frequency ofthe blade Ω_(o) in a band extended above and below the main resonancefrequency of the blade Ω_(o) in a percentage X of Ω_(o), saidfrequencies Ω₁, Ω₂, . . . Ω_(k) being comprised within said band.

In embodiments of the invention, said damping device is placed in ablade section where the twist is 0° in order to attenuate edgewisevibrations, or in a blade section with a predefined twist in order toattenuate both edgewise and flapwise vibrations, or in a blade sectionon the location of the second edgewise vibration modal shape to avoidexcitation of hybrid vibration modes. Hereby there are achievedefficient damping devices for wind turbines subjected to differentvibration modes.

In embodiments of the present invention, the number of said additionaldamping elements is two. It is considered that two additional dampingelements is a reasonable number of elements for achieving satisfactoryresults in wind turbine blades that on the other side facilitates theoptimization process for obtaining the parameters of modal mass m, modalstiffness k and damping coefficients c of the damping elements.

In embodiments of the present invention said percentage X is lesser than15% or even lesser than 11%. The corresponding bands are suitable bandsfor assuring for a wide range of blades a sufficient reduction of itsdynamic response irrespective of tolerance variations of blades of thesame model.

In embodiments of the present invention said percentage Y is lesser than40% or even lesser than 21%. It is considered that these limits may beachieved with suitable dimensioned damping elements, and shall cause anadequate load and deflections reduction, allowing an improvement of theblade performances.

Other features and advantages of the present invention will beunderstood from the following detailed description of illustrative andby no means limiting embodiments of its object in relation with theenclosed drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic section side view of a wind turbine.

FIG. 2 shows the typical dynamic response of a structure.

FIG. 3 shows the response of a structure with and without one damperelement tuned at the natural frequency,

FIG. 4 adds to FIG. 3 the response of a structure with two damperdevices having more than one damper element.

FIG. 5 shows a detailed energy diagram of a blade having a dampingdevice with K damping elements.

FIG. 6 shows the flapwise and edgewise directions.

FIG. 7 shows the frequency response of a blade with and without adamping device according to this invention.

FIG. 8 shows the reduction of loads due to damping coefficient rise.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As shown in FIG. 1, a typical wind turbine comprises a tower 3supporting a nacelle 8 housing a generator 9 for converting therotational energy of the wind turbine rotor into electrical energy. Thewind turbine rotor comprises a rotor hub 5 and, typically, three blades7. The rotor hub 5 is connected either directly or through a gearbox tothe generator 9 of the wind turbine for transferring the torquegenerated by the rotor 5 to the generator 9 and increase the shaft speedin order to achieve a suitable rotational speed of the generator rotor.

The response of a structure such as the blade of a wind turbine to anexcitation is a vibration with the features shown in the curve 31 ofFIG. 3 with maximum amplitude at the structure natural frequency Ω₀,which depends on the structure characteristics.

If the structure has a damping element properly adjusted to said naturalfrequency Ω₀, the response of the system is that shown by curve 33 inFIG. 3 with an amplitude attenuation at said natural frequency Ω₀ (thereduction depends on the damping capacity of said damping element) butwith an amplitude increase at frequencies Ω₁ and Ω₂, close to Ω₀.

Thus, the basic idea of the present invention is adding more dampingelements tuned at frequencies near to the natural frequency Ω₀ todiminish the amplitude of the response until a maximum is not exceededfor a certain range of frequencies as shown by curves 41 and 43 of FIG.4 with, respectively, maximum amplitudes of A₁ and A₂ corresponding todamping devices with several elements. The effect of the combination ofcertain number of damping elements does not produce exclusively a linealsuperposition of the effect of each one but also a synergic effect ofamplitude attenuation in a certain range of frequencies around thenatural frequency Ω₀ and a reduction of blade loads.

The determination of the number and properties, i.e. mass m, stiffness kand damping coefficient c of said damping elements, needed for achievingan adequate attenuation in the desired range of frequencies is made inan optimization process addressed to:

-   -   making an accurate adjustment of the resonance frequency of the        system;    -   dissipating the energy needed in order to achieve the required        load reduction.

Some theoretical explanation of how the energy is transferred from theblade to the damper for achieving a blade oscillation (and loads)reduction follows.

FIG. 5 show all the possible interactions between a blade B and thedamping elements D1, D2 . . . Dk when the blade is subjected to a PowerInput P_(B) that are considered in said optimization process:

-   -   Dissipated Power at the blade P_(d,B).    -   Transmitted Power between the blade B and the damping elements        D1, D2 . . . Dk: P_(B1), P_(B2), P_(Bk).    -   Received Power at the blade B from the damping elements D1, D2        Dk: P_(1B), P_(2B), P_(kB).    -   Transmitted and Received Power between damping elements D1, D2 .        . . . , P₁₂, P_(1k), P_(2k); P₂₁, P_(k1), P_(k2).    -   Dissipated Power at the damping elements D1, D2 . . . Dk:        P_(d,1), P_(d,2), P_(d,k).

Considering different generic subsystems i and j (i≠j) where i or jcorresponds to B for the blade, and 1 . . . k for the different dampingelements, the key parameters are:

-   -   Power dissipated by subsystem i: P_(i,d)=ωη_(i)E_(i)    -   Power interchanged between subsystems i and j:        P_(ij)=ωη_(ij)E_(i)−ωn_(ji)E_(j)    -   Reciprocity relation: n_(i)η_(ij)=n_(j)η_(ji)    -   Power balance for each subsystem:

$P_{i} = {P_{i,d} + {\sum\limits_{{j = 1},{i \neq j}}^{k}P_{ij}}}$

where:

-   P_(i) is the power introduced in each subsystem by an external    excitation. In this case this parameter represents the power    entering in the blade by the aerodynamic loads, P_(B).

P _(B)=∫₌₀ ^(=T) ^(∞) ∫₌₀ ^(=R) F _(a)(r,t)·δ{dot over (u)}(r,t)

where T_(ω), is the period of oscillation related to the eigenfrequencyω, F_(a) is the vector of the aerodynamic forces acting on the blade and{dot over (u)} is the vector of the modal displacements.

-   η_(i) is the internal losses factor of subsystem i. Represents the    percentage of losses produced when the entering power is converted    into energy in the blade or the damping elements. This factor    represents the damping level in each subsystem, so it can be    expressed as η_(i)=2γ_(i)=2C_(i)/C_(i,crit) and C_(i,crit)=2√{square    root over (k_(i)m_(i))}-   η_(i) is the coupling loss factor between subsystem i and j. The    power transmitted between interconnected subsystems can be    considered proportional to the energy in each subsystem (blade or    damping element). The η_(ij) parameters depend on the blade and    damping elements structural characteristics and can be obtained    analytically or by mathematical computational models (as FEM).-   n_(i) is the modal density for subsystem i, that represents the    average number of resonant eigenmodes in a subsystem i by frequency    band unity. In general for a blade is around 2 in the band of    interest (lower frequency modes), and 1 for the damping devices.

The parameters that are optimized to obtain the adequate damper devicefor a specific blade are therefore the interaction factors between bladeand damping elements and the interaction factors between the dampingelements and their internal losses factors, which are directly relatedto their corresponding damping factors γ_(i).

The damping device according to this invention comprises thereforeseveral damping elements whose parameters of mass m, stiffness k anddamping coefficient c are established according to the bladecharacteristics, in order to diminish main vibration modes. When theblade movement has a relevant contribution to a specific frequency bandand the damper device has been properly adjusted for that band, part ofthe blade energy is transferred to the damping device, so that the blademovement is attenuated. This happens because the damper dissipatesenergy of the movement, so it reduces the amplitude of the oscillationsof the blade.

Two main vibration modes occur on a wind turbine blade: flapwisevibration 25 and edgewise vibration 26 as shown in FIG. 6. Edgewisemovements are kept in the direction trailing edge—where the wind arrivesto the blade—to leading edge—where the wind leaves—. Flapwise vibrations25 are perpendicular to edgewise ones 26. This appointment is consideredbearing in mind the main zero-lift line 27 of the whole blade, not thezero-lift line 29 of the profile at a determined section (there is sometwist of the profiles when moving to the tip).

The damping device according to this invention is intended to act overthe amplitude of the blade vibration on a broad-band of frequenciesaround the main vibration modes, i.e. the flap and edge first vibrationmodes, that have the higher amplitude, for reducing the fatigue load.

Regarding the position of the damping device according to this inventionseveral criteria shall be taken into account such as space limitationsinside the blade, inertia loads added to the blade depending on the massand position of the damping device, desired effects over the vibrationmodes and secondary effects due to the installation of the dampingdevice.

Suitable positions of the damping device can be the following:

-   -   A blade section where the twist, i.e. the angle between the        zero-lift profile line 29 and the main zero-lift line 27, is 0°        in order to attenuate edgewise vibrations.    -   A blade section with a predefined twist in order to attenuate        both edgewise and flapwise vibrations, or only edgewise ones,        depending on requirements of each blade.    -   A blade section on the location of the second edgewise vibration        modal shape to avoid excitation of hybrid vibration modes. This        section is usually located in the second third of the blade.

In a preferred embodiment of the present invention the damping devicecomprise a first damping element tuned at the main resonance frequencyof the blade Ω_(o) and 2 additional damping elements tuned atfrequencies Ω₁, Ω₂,

The response of the blade near to main vibration frequencies with andwithout a damper device according to this preferred embodiment is thatrepresented by, respectively, curves 53 and 51 in FIG. 7. Therefore thedamper device allows an amplitude reduction below a predetermined limitA_(max) in the desired frequency band Ω_(a)−Ω_(b), where Ω_(a)=Ω_(o)−X%*Ω_(o) and Ω_(b)=Ω_(a)+X %*Ω_(o)

Preferably said percentage X % is lesser than 15% or even that 11% andthe bands so defined cover a frequency band wide enough to cope up withfabrication deviations and give to the damping device a sufficientmargin to act properly on different ambient conditions. Within saidbands the frequencies Ω₁, Ω₂, of the additional damping elements shallbe properly selected for achieving the synergic effect above-mentioned,i.e. acting as a broad-band damper device instead of the sum of threeindividual damping elements.

Preferably said predetermined limit of A_(max) is lesser than the 40% oreven the 25% of the amplitude A_(o) at the main resonance frequency ofthe blade Ω_(o) without any damping element.

The reduction achieved in the amplitude of the vibration avoids, on oneside, structural risks for the blade and, on the other side, allows theblade to cope with higher loads. This is based on the fact that asblades are flexible components, loads have always a queasy-steadycomponent (related to wind speed among other parameters) and a dynamiccomponent (e.g. coming from air gusts) that is directly related to totaldamping (structural and aerodynamic). If damping is increased, thedynamic loading can be reduced, that allows increasing queasy-steadyloading (allowable maximum wind speed) without exceeding total loads onthe blade. Alternatively, just increasing damping maintaining thedimensioning wind conditions allows optimizing the blade structure(weight and cost) as aforementioned.

The effect of the damping device according to this invention is not onlyreducing blade deflections at certain frequencies. Also loads over theblade lessen, so adding dampers is a clear improvement. FIG. 8 showsreduction of loads L over a blade for different simulations S, withcases 63, 65, 67 increasing damping coefficient with respect to the case61 of a blade without a damping device.

The above-mentioned quantitative information regarding the preferredembodiment of the damping device with three damping elements tuned atfrequencies Ω₀, Ω₁, Ω₂ has been obtained in a detailed parametric studyand has been verified by experimental tests.

A damping device according to this invention is a suitable dampingdevice for stall regulated wind turbines during normal operation and tostall regulated and pitch regulated wind turbines when parked or inidling situations with high incidence angles.

Although the present invention has been fully described in connectionwith preferred embodiments, it is evident that modifications may beintroduced within the scope thereof, not considering this as limited bythese embodiments, but by the contents of the following claims.

1. Wind turbine comprising one or more blades, characterized in thateach blade incorporates a damping device comprising a first dampingelement tuned at the main resonance frequency of the blade Ω_(o) and Kadditional damping elements tuned at frequencies Ω₁, Ω₂, . . . Ω_(k), sothat in the event of blade vibrations its amplitude A is reduced in apercentage Y of the amplitude A_(o) at the main resonance frequency ofthe blade Ω_(o) in a band extended above and below the main resonancefrequency of the blade Ω_(o) in a percentage X of Ω_(o), saidfrequencies Ω₁, Ω₂, . . . Ω_(k) being comprised within said band. 2.Wind turbine according to claim 1, wherein said damping device is placedin a blade section where the twist is 0° in order to attenuate edgewisevibrations.
 3. Wind turbine according to claim 1, wherein said dampingdevice is placed in a blade section with a predefined twist in order toattenuate both edgewise and flapwise vibrations.
 4. Wind turbineaccording to claim 1, wherein said damping device is placed in a bladesection on the location of the second edgewise vibration modal shape toavoid excitation of hybrid vibration modes.
 5. Wind turbine according toany of claims claim 1, wherein the number of said additional dampingelements is two.
 6. Wind turbine according to claim 5, wherein saidpercentage Xis lesser than 15%.
 7. Wind turbine according to claim 6,wherein said percentage Xis lesser than 11%.
 8. Wind turbine accordingto any of claims claim 5, wherein said percentage Y is lesser than 40%.9. Wind turbine according to claim 8, wherein said percentage Y islesser than 25%.
 10. Wind turbine according claim 1, wherein itsregulation system is a stall regulation system.
 11. Wind turbineaccording to claim 1, wherein its regulation system is a pitchregulation system.